Maplestory at custom resolutions

How is that any different than the current resolutions Maple has?

You're right, it isn't any different. Different resolutions is a part of Nexon's power creep. They designed multiple pieces of high end content and then deliberately made them much easier through letting you view the game in a higher resolution. Nexon is more than fully aware there are huge in game advantages to seeing the game in the biggest resolution possible and that is exactly why they took this registry edit away.